Representatives of the ATF and the Texas Fire Marshall provided an update on their joint investigation into the fertilizer plant explosion in West Texas. The short story is that the cause of the fire is undetermined. The long story is that the investigation has been as massive as was the explosion.

At 7:29 pm on April 17, something caught fire in the seed and fertilizer building at the plant. Three minutes later, the fire department was dispatched; by 7:38 firefighters were on-scene. Three minutes later, more firefighters were called out.

At 7:51, the fire was hot enough to change the sensitivity of the ammonium nitrate stored in the building in a tall column of wooden bins. Something hit the unstable compound — a piece of equipment, or some debris — and triggered a small explosion. That explosion created enough additional heat and enough shock to set off a far larger blast a few milliseconds later. Investigators know there were two explosions because they were separately recorded on a seismograph at a nearby college. An estimated 28 to 34 tons of ammonium nitrate exploded, with the force of 15,000 to 20,000 pounds of TNT. An additional 20 to 30 tons in the building didn't go off, nor did 100 tons in a nearby railcar.

The blast killed twelve of the first responders at the scene, and three others elsewhere. It left a crater 93 feet wide and ten feet deep. Debris was scattered over a 37-block area, and pieces were found 2.5 miles away. Over 104 people from the two agencies spent 30 days going over the scene of the blast, excavating and mapping the crater, generating nearly 300 leads, interviewing 500 people. They combed an area covering about 14 acres. At one point, investigators looking for evidence sifted through an entire silo of corn — 300,000 pounds — by hand.

Despite that effort, what caused the fire isn't clear. Investigators have rules out a number of causes. It wasn't due to an earlier fire rekindling, as had been speculated. It wasn't spontaneous ignition from equipment in the plant or its 480-volt electrical system. It wasn't weather or smoking.

There are three possibilities. The first is that a second electrical system in the plant, its 120-volt system, malfunctioned. The second is that a battery failed on a golf cart kept in the room where the fire started. There have been cases in which such failures have started fires, and investigators have only found two small pieces of the cart, impairing their ability to evaluate it.

And then, of course, there's arson. The investigators pointed out that their investigation led them to Bryce Reed, an EMS worker arrested for possession of bomb-making materials. "At this time," a representative of the ATF told reporters, "authorities will not speculate whether that possession has any connection" to the explosion at the fertilizer plant. Beyond that, there was no indication of who might have intentionally set the fire.

The investigation in West, at the scene, is over. Now the investigation, which remains open, turns largely to analyzing the evidence they already have.